This invention relates in general to the construction of plasticizing devices and, in particular, to a new and useful screw assembly of a molding machine for plasticizable masses, particularly, for plastics or rubber, which includes a passage portion of the screw which has a bore therethrough defining a mixing passage with mixing elements therein for effecting the static intermixing of the material as it passes therethrough, and which is included as a portion of the screw in such a manner that a communicating passage from the periphery of the screw upstream feeds the material into the passage, and a communicating passage downstream permits the discharge of the material after it is mixed.